Android uses a modified version of the Linux kernel.

when people say linux phones, the point is not the kernel but believing it could have the same open ecosystem as the linux PC. no forced lockdown, no spyware, open source and in turn extensible system software, where its not the oligarchs who dictate. other then the first point these can be told about mostly none of the current phones, and even the first point is going away recently, despite that being the only way to get rid of the preinstalled, google mandated (and sometimes additional) malware.